Electrochemical methods for generation of a biological proton motive force and pyridine nucleotide cofactor regeneration

ABSTRACT

Disclosed are methods using neutral red to mediate the interconversion of chemical and electrical energy. Electrically reduced neutral red has been found to promote cell growth and formation of reduced products by reversibly increasing the ratio of the reduced:oxidized forms of NAD(H) or NADP(H). Electrically reduced neutral red is able to serve as the sole source of reducing power for microbial cell growth. Neutral red is also able to promote conversion of chemical energy to electrical energy by facilitating the transfer of electrons from microbial reducing power to a fuel cell cathode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisionals Ser. Nos. 60/092,190 and 60/092,191, both filed Jul. 9, 1998, which are incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with U.S. Government support in the form of the United States Department of Energy grant DE-FG02-93ER20108. The United States may have certain rights in this invention.

BACKGROUND OF THE INVENTION

Microbial fermentation and biotransformation reactions are being employed with increasing frequency in the production of a number of commercially and industrially important products. There is also growing interest in developing alternative energy sources through microbial fermentation of waste materials. The economic feasibility of these processes depends on maximizing the efficiency of the fermentation or biotransformation reactions.

Bacterial species are able to use various energy sources, including light and diverse organic and inorganic chemicals, for growth and metabolism. These energy sources are used to produce an electrochemical gradient that provides an electron donor for metabolism and allows maintenance of a membrane potential and proton motive force. The energetics of living systems are driven by electron transfer processes in which electrons are transferred from a substrate, which is thereby oxidized, to a final electron acceptor, which is thereby reduced.

In microbial metabolism, the energy produced from the driving force of electrons is directly proportional to the potential energy difference (ΔE_(o)′) between the initial electron donor (the first biochemical dehydrogenating reaction) and final electron acceptor (e.g., the final biochemical hydrogenating reaction).

Certain microorganisms (e.g., Escherichia and Actinobacillus) are able to grow using H₂ as an electron donor to reduce fumarate into succinate in an anaerobic respiration process. These bacteria obtain free energy and reducing power from the electron driving force generated by the E_(o)′ difference between the coupled oxidoreduction half reactions of [2H⁺/H₂] and [fumarate/succinate].

Methanogens are strict anaerobic archea that can couple H₂ or HCOOH oxidation to CO₂ reduction into methane. Methanogenesis produces less free energy than other anaerobic respiration processes (e.g., fumarate, nitrate, or sulfate reduction) because the E_(o)′ difference between the half oxidation reduction reactions of [2H⁺/H₂] and [CO₂/CH₄] is relatively small.

Hydrogen oxidation by microbial hydrogenases can be coupled to reduction of various biological electron carriers including NAD⁺, cytochromes, and quinones or to certain artificial redox dyes, such as methyl-viologen and neutral red (NR) (Annous et. al., 1996, Appl. Microbiol. Biotechnol. 45:804-810, Kim et al., 1992, J. Microbiol. Biotechnol. 2:248-254). The effect of redox dyes, with or without electrochemical reduction systems, on metabolite patterns and H₂ production has been examined in several microbial processes, including the glutamate (Hongo et. al., 1979, Agric. Biol. Chem. 43:2083-2986), butanol (Girbal et. al., 1995, Microbiol. Rev. 16:151-162, and Kim et. al., 1992, J. Microbiol. Biotechnol, 2:268-272), and butyrate (Shen et. al., 196, Appl. Microbial, Biotechnol, 45:355-362) fermentations.

The specific activities of redox enzymes involved in bacterial catabolism, such as hydrogenase or fumarate reductase, can be measured using their in vivo electron carriers (e.g., NAD or menanquinone) or with artificial redox dyes (e.g., benzyl viologen) (Cecchini et. al., 1986, Proc. Natl. Acad. Sci. USA 83:8898-8902, Dickie et. al., 1979, Can. J. Biochem., 57:813-821, Kemner et. al., 1994, Arch. Microbial., 161:47-54, Petrov et. al., 1989, Arch. Biochem. Bio-phys. 268:306-313, and Wissenbach et. al., 1990, Arch. Microbiol. 154:60-66). Bacteria that produce succinic acid as a major catabolic end product (e.g., E. coli, Wolinella succinogenes and other species) have a fumarate reductase (FRD) complex that catalyzes fumarate-dependent oxidation of menaquinone. This reaction is coupled to the generation of a transmembrane proton gradient that is used by the organism to support growth and metabolic function (Kortner et. al., 1992, Mol. Microbiol. 4:855-860 and Wissenbach et. al., 1992, Arch. Microbial. 158:68-73). The fumarate reductase of E. coli is composed of four nonidentical subunits: FRDA, FRDB, FRDC, and FRDD. The subunits are arranged in two domains: (i) the FRDAB catalytic domain and the FRDCD membrane anchor domain, which is essential for electron transfer and proton translocation reactions involving menaquinone (Cecchini et. al., 1995, J. Bacteriol. 177:4587-4592, Dickie et. al., 1979, Can. J. Biochem., 57:813-821, and Westenberg et. al., 1990, J. Biol. Chem. 265:19560-19567). Subunits FRDA and FRDB retain catalytic activity in solubilized membrane preparations.

Electrochemical techniques employing redox dyes are useful for investigating the oxidation-reduction characteristics of biological systems and provide information about biological energy metabolism (Moreno et. al., 1993, Eur. J. Biochem. 212:79-86 and Sucheta et. al., 1993, Biochemistry 32:5455-5465). Redox dyes that are useful in bioelectrochemical systems must easily react with both the electrode and the biological electron carriers. Many biological electron carriers, such as NAD (Miyawaki et. al., 1992, Enzyme Microb. Technol. 14:474-478 and Surya et. al., 1994, Bioelectrochem. Bioenerg. 33:71-73), c-type cytochromes (Xie et. al., 1992, Bioelectrochem. Bioenerg. 29:71-79), quinones (Sanchez et. al., 1995, Bioelectrochem. Bioenerg. 36:67-71), and redox enzymes, such as nitrite reductase (White et. al., 1987, Bioelectro-chem. Bioenerg. 26:173-179), nitrate reductase (Willner et. al., 1992, Bioelectrochem. Bioenerg. 29:29-45), fumarate reductase (Sucheta et. al., 1993, Biochemistry. 32:5455-5465), glucose-6-phosphate dehydrogenase (Miyawaki et. al., 1992, Enzyme Microb. Technol. 14:474-478), ferredoxin-NADP reductase (Kim et. al., 1992, J. Microbiol. Biotechnol. 2:2771-2776) and hydrogenase (Schlereth et. al., 1992, Bioelectrochem. Bioenerg. 28:473-482) react electrochemically with the redox dyes.

Certain redox dyes with lower redox potentials than that of NAD, such as methyl viologen (MV) (Kim et. al., 1988, Biotechnol. Lett. 10:123-128, Pequin et. al., 1994, Biotechnol. Lett. 16:269-274, and White et. al., 1987, FEMS Microbiol. Lett. 43:173-176), benzyl viologen (Emde, et. al., 1990, Appl. Environ. Microbiol. 56:2771-2776), and neutral red (NR) (Girbal et. al., 1995, FEMS microbial. Rev. 16:151-162 and Kim et. al., J. Biotechnol. 59:213-220) have been correlated with alterations in the rate of biological redox reactions in vivo. Hongo and Iwahara (Hongo et. al., 1979, Agric. Biol. Chem. 43A:2075-2081 and Hongo et. al., 1979, Agric. Biol. Chem. 43B:2083-2086) discovered that including redox dyes with low ΔE_(o)′ values (e.g., MV, benzyl viologen and NR) in bacterial fermentation conducted under cathodic reduction conditions was correlated with an increase in L-glutamate yield (about 6%). In the method of Hongo and Iwahara, a platinum electrode was used to deliver electricity at a level that was sufficiently high to generate hydrogen from water. Therefore, the source of increased reducing power in the method of Hongo and Iwahara is not known, nor was the mechanism by which the tested dyes affect fermentation characterized. Addition of NR to acetone-butanol fermentations is correlated with decreased production of acids and H₂, and enhanced production of solvent (Girbal et. al., 1995, FEMS Microbiol. Rev. 16:151-162 and Kim et. al., 1992, J. Microbiol. Biotechnol. 2:2771-2776), an effect that was further enhanced under electroenergized fermentation conditions (Ghosh et. al., 1987, abstr. 79. In Abstracts of Papers, 194th ACS National Meeting. American Chemical Society). Viologen dyes have been used as electron mediators for many electrochemical catalytic systems using oxidoreductases in vitro and in vivo (James et. al., 1988, Electrochem. Bioenerg. 20:21-32, Kim et. al., 1988, Biotechnol. Lett. 10:123-128, Moreno et. al., 1993, Eur. J. Biochem. 212:79-86, Schlereth et. al., 1992, Bioelectrochem. Bioenerg. 28:473-482, and White et. al., 1987, FEMS Microbiol. Lett. 43:173-173). An electrochemical system was used to regenerate reduced iron for growth of Thiobacillus ferrooxidans on electrical reducing power (Robinson et. al., 1982, Can. J. Biochem. 60:811-816).

It may be possible to control or alter metabolism by linking biochemical processes to an external electrochemical system. Linking biochemical and electrochemical systems may allow the use of electricity as a source of electrons for bacterial growth and in vivo or in vitro fermentation or biotransformation reactions.

A reversible biochemical-electrochemical link may allow conversion of microbial metabolic or enzyme catalytic energy into electricity. Biofuel cells in which microbial energy is directly converted to electrical energy using conventional electrochemical technology have been described (Roller et. al., 1984, J. Chem. Tech. Biotechnol. 34B:3-12 and Allen et. al., 1993, Appl. Biochem. Biotechnol. 39-40:27-40). Chemical energy can be converted to electric energy by coupling the biocatalytic oxidation of organic or inorganic compounds to the chemical reduction of the oxidant at the interface between the anode and cathode (Willner et. al., 1998, Bioelectrochem. Bioenerg. 44:209-214). However, direct electron transfer from microbial cells to electrodes has been shown to take place only at very low efficiency (Allen et. al., 1972, J. R. Norris and D. W. Ribbons (eds.), Academic Press, New York, 6B:247-283).

The electron transfer efficiency can be improved by using suitable redox mediators (Bennetto et. al., 1985, Biotechnol. Lett. 7:699-105), and most of the microbial fuel cells studied employed electron mediators such as the redox dye thionin (Thurston et. al., 1985, J. Gen. Microbiol. 131:1393-1401). In microbial fuel cells, two redox couples are required for: (1) coupling the reduction of an electron mediator to bacterial oxidative metabolism; and (2) coupling the oxidation of the electron mediator to the reduction of the electron acceptor on the cathode surface (where the electron acceptor is regenerated by atmospheric oxygen) (Ardeleanu et. al., 1983, Bioelectrochem. Bioenerg. 11:273-277 and Dealney et. al., 1984, Chem. Tech. Biotechnol. 34B:13-27).

The free energy produced by either normal microbial metabolism or by microbial fuel cell systems is mainly determined by the potential difference (ΔE_(o)′) between the electron donor and acceptor according to the equation, −ΔG=nFΔE_(o) in which ΔG is the variation in free energy, n is the number of electron moles, and F is the Faraday constant (96,487 J/volt) (Dealney et. al., 1984, Chem. Tech. Biotechnol. 34B:13-27). Coupling of the metabolic oxidation of the primary electron donor (NADH) to the reduction of the final electron acceptor (such as oxygen or fumarate in bacterial respiration systems) is very similar to the coupling of electrochemical half-reaction of the reductant (electron donor) to the half reaction of the oxidant (electron acceptor) in a fuel cell or battery system (Chang et. al., 1981, 2nd ed., Macmillan Publishing, New York). Biological reducing power sources such as NADH (E_(o) ′=−0.32 volt), FdH₂ (E_(o) ′=−0.42 volt), or FADH₂ (E_(o) ′=−0.19 volt) with low redox potentials can act as reductants for fuel cells, but they are not easily converted to electricity because the cytoplasmic membrane must be non-conductive to maintain the membrane potential absolutely required for free energy (i.e., ATP) production (Thauer et. al., 1997, Bacteriol. Rev. 41:100-180).

For electron transfer to occur from a microbial electron carrier to an electrode, an electron mediator is required (Fultz et. al., 1982, Anal. Chim. Acta. 140:1-18). Allen et al. (1993, Appl. Biochem. Biotechnol. 39-40:27-40) reported that the reducing power metabolically produced by Proteus vulgaris or E. coli can be converted to electricity by using electron mediators such as thionin. Tanaka et al (1985, Chem. Tech. Biotechnol. 35B:191-197 and 1988, Chem. Tech. Biotechnol. 42:235-240) reported that light energy can be converted to electricity by Anabaena variabilis using HNQ as the electron mediator. Park et al (1997, Biotech. Technig. 11:145-148) confirmed that viologen dye cross-linked with carbon polymers and adsorbed to Desulfovibro desulfuricans cytoplasmic membranes can mediate electron transfer from bacterial cells to electrodes or from electrodes to bacterial cells.

There remains a need in the art for improved, more efficient methods for converting metabolic reducing power to electrical energy, and for converting electrical energy to metabolic reducing power.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention is a method of promoting reductive processes in a bioreactor system comprising the steps of (a) providing an electrochemical bioreactor system having a cathode compartment equipped with a cathode and an anode compartment equipped with an anode, the cathode and anode compartment being separated by a cation selective membrane, wherein the cathode and anode are connected by a conductive material to a power supply; (b) placing a suitable amount of neutral red and a biological catalyst in the cathode compartment.

Another aspect of the invention is a method for generating electricity using a biological system comprising the steps of (a) providing an electrochemical fuel cell system comprising an anode compartment and a cathode compartment separated by a cation-selective membrane, wherein each compartment is equipped with an electrode, wherein the electrodes are connected by a wire to a multimeter; (b) placing an anolyte in the anode compartment, the anolyte comprising a suitable concentration of neutral red and a biological catalyst selected from the group consisting of bacteria, archea, plant cells, and animal cells; (c) placing a suitable catholyte in the cathode compartment; and (d) allowing the neutral red-mediated conversion of chemical reducing power to electricity.

It is an object of the invention to provide methods that allow the interconversion of biochemical reducing power (e.g., NADH), biological energy (ATP), and electrical energy in an electrochemical bioreactor or fuel cell.

It is a further object of the invention to provide an economical method of promoting cell growth or production of desired products using electrically reduced neutral red.

Another object of the invention is to provide a method for converting biological reducing power into electricity.

It is an advantage of the present invention that electrical energy may be used to promote cell growth or fermentation or enzymatic transformation in the presence of neutral red.

Another advantage of the invention is that neutral red promotes the generation of electrical energy from waste material comprising mixed bacterial populations.

Other objects, features, and advantages of the present invention will be apparent on review of the specification and claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic diagram of a microbial fuel cell using neutral red (NR) as an electronophore.

FIG. 2 shows the current production from NADH oxidation in a chemical fuel cell with NR (A) or thionin (B) as the electron mediator.

FIG. 3 is a cyclic voltammogram obtained with a glassy carbon electrode on successive cycles following introduction of the electrode into a 100 μM NAD⁺ solution.

FIG. 4 shows the current and potential obtained in a glucose fuel cell using E. coli K-12 resting cells and neutral red or thionin.

FIG. 5 shows the electrical current and potential levels obtained using A. succinogenes growing or resting cells.

FIG. 6 shows the current and potential produced in a glucose (3 g/L) fuel cell using anaerobic sewage sludge as catalyst and NR (100 μM) as the electronophore.

FIG. 7 is a proposed model of the energy flow in cells under normal (A) or electrogenic (B) glucose metabolism.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods for achieving the efficient interconversion of chemical and electrical energy using neutral red. One aspect of the invention is a method for using electrical energy as a source of reducing power in fermentation or enzymatic reactions. Another aspect of the invention includes a method of using neutral red and cells or enzymes to produce electricity.

The invention is based on the discovery that the use of neutral red in methods directed toward regulating electron flow in biological systems offers a number of surprising advantages, which are disclosed in related U.S. Ser. Nos. 60/092,190 filed Jul. 9, 1998 and 60/092,191; filed Jul. 9, 1998 Park and Zeikus, J. Bacteriol. 181:2403-2410, 1999; and Park et al. Appl. Environ. Microbiol. In press, all of which are incorporated by reference in their entirety.

A critical factor for the control of end product yields in fermentation or enzymatic biotransformation reactions is regulation of electron distribution through the NADH/NAD⁺ ratio. If additional reducing power (e.g., H₂ or electrochemically produced reducing equivalents) is supplied to bacteria, an increase in the NADH/NAD⁺ ratio and metabolism may be expected. However, efficient transfer of electrons from electricity to NAD⁺ requires a suitable electron mediator.

As discussed in detail in U.S. Ser. Nos. 60/092,190 and 60/092,191, neutral red was discovered to be a particularly good electron mediator for use in the interconversion of electricity and metabolic reducing power in electrochemical bioreactor systems. Neutral red is able to form a reversible redox couple at the electrode and has a highly negative E_(o) ′. The E_(o) ′ value for neutral red is very similar to that of physiological electron carriers in the electron transport chain, including, for example, NADH. The ability of neutral red to accept electrons from far up the electron transport chain enhances electricity production in biofuel cell systems. Neutral red is soluble at a neutral pH, it is stable in both its oxidized and reduced forms, it does not decompose during long-term redox cycling.

As disclosed in U.S. Ser. Nos. 60/092,190 and 60/092,191, neutral red is relatively nontoxic, and can be easily adsorbed on the cytoplasmic membrane of the cells under study, where it functions as an electronophore, or electron shuttle, for electron transfer across the cytoplasmic membrane. Neutral red was demonstrated to function as an electron mediator in reversible oxidation or reduction of compounds and to substitute for menaquinone in the cell membrane. Surprisingly, electrically reduced neutral red promotes growth, proton translocation and metabolite production in cells even in the absence of other sources of reducing power.

One aspect of the present invention provides a method for promoting reductive processes in a bioreactor system comprising the steps of (a) providing an electrochemical bioreactor system having a cathode compartment equipped with a cathode and an anode compartment equipped with an anode, the cathode and anode compartment being separated by a cation selective membrane, wherein the cathode and anode are connected by a conductive material to a power supply; and (b) placing a suitable amount of neutral red and a biological catalyst in the cathode compartment.

Preferably, the biological catalyst is selected from the group consisting of microbial cells, plant cells, animal cells, isolated intact cytoplasmic membranes, solubilized cytoplasmic membranes, and an enzyme having NADH or NADPH a cofactor. To maximize the efficiency of the interconversion of biochemical and electrical energy, the biological catalyst is immobilized on the cathode.

In a preferred embodiment, the method of the invention further comprises the steps of (c) placing an anolyte solution in the anode compartment; (d) delivering to the cathode an electric current of suitable strength to cause reduction of at least a portion of oxidized neutral red in the cathode compartment; and (e) allowing the reduced neutral red to transfer electrons to an oxidized substrate or an electron carrier.

The method of the invention is very versatile, in that it can be adapted for use with any number of biological catalysts, including microbial, plant, or animal cells, isolated intact cell membranes, solubilized cytoplasmic membranes, or a preparation of an enzyme that uses NADH or NADPH as a cofactor. Most conveniently, the biological catalyst comprises substantially pure or mixed cultures of cells, or an enzyme preparation. Preferably, the biological catalyst is capable of promoting the reduction of an oxidized substrate to a commercially or industrially important product, such as succinate, methane, or alcohols.

When whole cells are used as the biocatalyst, electrically reduced neutral red promotes cell growth or formation of a reduced product by chemical reduction of an NAD⁺ or NADP⁺ cofactor, or by serving as an electronophore. Preferably, the bioreactor system is one in which the electrically reduced neutral red promotes cell growth, ATP synthesis, or formation of a reduced product by chemical reduction of an NAD⁺ or NADP⁺ cofactor or by functioning as an electronophore.

In the examples below, electrically reduced neutral red is shown to promote the reduction of fumarate to form succinic acid in fermentation reactions using Actinobacillus succinogenes in a bioreactor system. Because succinic acid is an important fermentation product having many industrial uses, there is interest in developing a more efficient fermentation process with enhanced succinic acid yields.

It was discovered that including electrically reduced neutral red during growth of A. succinogenes on glucose medium in a bioreactor system promotes fumarate reduction by chemically reducing NAD⁺. Furthermore, neutral red promotes succinic acid production through its function as an electron mediator and electronophore. The electrical reduction of neutral red (E_(o) ′=−0.325 volt) is chemically linked to NAD⁺ reduction, and it is biochemically linked to generation of a proton motive force and succinate production. Neutral red appears to function by replacing menaquinone (E_(o) ′=−0.073 volt) in the membrane bound fumarate reductase complex. Preferably, the reduced neutral red is able to increase cell growth by at least 10%, 20%, or even as much as 40% or more, relative to a comparable bioreactor system lacking neutral red. Electrically reduced neutral red is able to increase glucose or fumarate consumption by at least 25%, 50%, or 100% or more. Succinate production is increased by about 10% or even as much as 25% or more, relative to the production levels observed in a comparable bioreactor system lacking neutral red.

Similarly, electrically reduced neutral red is able to substitute for H₂ in promoting the growth of methanogenic bacteria and the reduction of CO₂ to methane by methanogenic archea. Preferably, the method of the invention increases growth of archea or methane production by at least about 25%, 50%, 100% or even as much as 300% or more.

It is reasonable to expect that the method of the present invention may be used with a wide range of biocatalysts to promote cell growth or the formation of reduced products in electrochemical bioreactor systems. It is envisioned that the method can be used with a variety of bacteria, archea, plant cells or animal cells.

It is expected that enzyme preparations may also be used in the practice of the invention. A desired enzyme may be partially purified using standard methods known to one of ordinary skill in the art. The enzyme may be isolated from its native source or from a transgenic expression host, or obtained through a commercial vendor.

Useful enzymes include any enzyme that can use reducing power from electrically reduced neutral red to form a desired reduced product, or which can transfer reducing power to neutral red and form a desired oxidized product. Most commonly, this reduction is mediated by NADPH or NADH. It is reasonably expected that any oxidoreductase may be used in the practice of the invention. For example, isolated alcohol dehydrogenases could be used in a bioreactor system comprising electrically reduced neutral red, NADP⁺ or NAD⁺, and a ketone, aldehyde or carboxylic acid that can serve as a substrate for the enzyme to form a more reduced end product such as an alcohol. Another example of a useful enzyme is carboxylic acid reductase, which uses NADPH and ATP to convert a carboxylic acid to reduced products (U.S. Pat. No. 5,795,759, herein incorporated by reference). One skilled in the art would appreciate that most enzyme-catalyzed reactions are reversible, and that there may be applications in which one would wish to use an oxidoreductase to obtain a desired oxidized substrate by the method of the present invention.

In the electrochemical bioreactor used in the present invention, the biocatalyst and neutral red are preferably immobilized on the cathode. In the case of whole cell biocatalysts, self-immobilization on a fine woven graphite felt electrode was found to take place. Immobilization of the biocatalyst may be achieved using any suitable method. Numerous techniques for immobilizing biocatalysts are known to the art (for example, see Woodward and Spokane, Analytical Enzymes: Biosensors in Industrial Enzymology, 2d Edition, p. 51-59, incorporated by reference herein). One wishing to immobilize a biocatalyst in the practice of the present invention could do so placing the biocatalyst, neutral red, and pyridine nucleotide cofactor between an electrode and an outer membrane (e.g., a polymer membrane) such that the biocatalyst, cofactor, and neutral red are sandwiched between the electrode and membrane. Alternatively, biocatalyst, neutral red, and pyridine nucleotide cofactor could be embedded in a matrix polymer and coated onto the electrode.

One of ordinary skill in the art wishing to practice the present invention could readily prepare an electrochemical bioreactor or fuel cell using the teachings disclosed herein. It should be appreciated that certain modifications to the disclosed bioreactors and fuel cells are well within the ability of one skilled in the art.

Catholytes and anolytes that may be used in electrochemical bioreactors or in fuel cells are provided in the examples. Catholytes that have been found to be suitable in electrochemical bioreactors include bacterial growth media or a phosphate buffer (50-100 mM, pH 7.0-7.2). Other suitable catholyte buffers for used in an electrochemical bioreactor include any catholyte that is nondenaturing to cells or enzymes.

A phosphate buffer comprising saline has been found to be suitable for use in an electrochemical bioreactor (100 mM sodium phosphate (pH 6.0) and 100 mM NaCl. A suitable anolyte may include any anolyte that is nondenaturing to cells or enzymes.

For a fuel cell, neutral red (100 μM) and a bacterial cell suspension in 50 mM phosphate buffer (pH 7.0) was found to be a suitable anolyte, with 100 mM phosphate buffer (pH 7.0) and 50 mM ferricyanide as the catholyte.

In both the electrochemical bioreactor systems and the fuel cell system described in the examples, the cathodic and anodic compartments were separated by a Nafion cationic selective membrane septum that allows the passage of protons and cations only. A suitable membrane for separating the cathodic and anodic compartments can be any membrane that allows transfer of only protons or cations across the membrane.

In the electrochemical bioreactor systems described in the examples below, the electrodes were made from fine woven graphite felt. The woven graphite felt offers the advantage of providing a large surface area electrode that permits immobilization of the biocatalyst over a large area. However, other materials may be suitable for electrodes, including conductive polymers and metallic materials.

The electrodes were connected to a power source or to a multimeter using a platinum wire. Other materials suitable for connecting the electrodes to the power source or multimeter include conducting polymers or metallic materials.

In the electrical bioreactors described below, the current between the anode and cathode was between about 0.4 and about 2.0 mA, with the voltage being about 1.5 V. It is envisioned the present invention could be practiced using currents of from about 0.004 to about 200 mA.

In the fuel cell system, the resistance from the anode and cathode was about 1,000 ohms. It is envisioned that resistances of from about 10 to about 10,000 ohms could be used in the practice of the invention.

Neutral red was included in the catholyte of electrochemical bioreactors and in the anolyte of fuel cell systems at a concentration of about 100 μM. It is expected that neutral red concentrations of between about 1 and 1000 μM would be suitable in the practice of the invention.

Neutral red can also be used as an electron mediator in the conversion of energy derived from the metabolism of growing or resting bacterial cells to electricity.

Using Actinobacillus succinogenes 130Z growing cells in a fuel cell system that had neutral red as the electron mediator and ferricyanide as the electron acceptor, the maximum current produced using was 2.17 mA, and the potential was <100 mV in a closed circuit configuration. After 20 h cultivation, the fuel cell system was converted from a closed to an open circuit system. The potential rapidly reached the theoretical maximum value of 0.685 volt (i.e. the redox potential difference between NR).

A comparison of the efficacy of NR and thionin as electron mediators made using A. succinogenes resting cells as the catalyst revealed that much more electricity was produced with NR than with thionin as the electron mediator. When NADH, NR, and ferricyanide were used as the electron donor, electron mediator, and electron acceptor, respectively, the current produced was proportional to NADH concentration. In a system that employed Escherichia coli K-12 as the catalyst, the currents and voltages produced were similar to those obtained using A. succinogenes as the catalyst. The current and voltage were found to increase with increasing glucose concentrations.

Anaerobic sewage sludge was also used as the catalyst in a fuel cell system. The voltage and current produced in fuel cells using sewage sludge as the catalyst were comparable to those produced using E. coli and Actinobacillus, and they were stable for 120 h in a closed circuit system with a 2.2 K ohms external resistance.

It is expected that growing or resting cells of types other than those described in the examples can be used as catalysts in a fuel cell system to generate electricity by the method of the present invention. Depending on the particular cell chosen as a biocatalyst, reducing power used in the generation of electricity may include light, inorganic compounds, or organic compounds, or any other energy source that cells are able to use for growth or metabolism.

It is envisioned that the neutral red-mediated interconversion of biochemical and electrical energy may be adapted for use in a number of different applications. For example, neutral red oxidoreduction can be used to detect electrical levels in biosensor systems using whole cells or enzymes.

Accordingly, the invention includes a method for detecting the presence of a specific organic or inorganic test compound in a sample comprising the steps of (a) providing biosensor comprising an electrochemical fuel cell system having an anode compartment and a cathode compartment separated by a cation-selective membrane, wherein each compartment is equipped with an electrode, wherein the electrodes are connected by a wire to a multimeter; (b) placing an anolyte in the anode compartment, the anolyte comprising the sample, a suitable concentration of neutral red, and a biological catalyst comprising microbial cells and an enzyme, wherein the biological catalyst is able to oxidize the test compound; (c) placing a suitable catholyte in the cathode compartment; and (d) allowing oxidation of at least a portion of any test compound present in the sample and reduction of at least a portion of oxidized neutral red; (e) allowing the transfer of electrons from reduced neutral red to the cathode; (f) detecting the generation of an electrical current.

In cell or enzyme biosensors known to the art, the presence of a chemical (e.g., glucose) is detected using an enzyme (glcuose oxidase) in a membrane-based electrode system. In the example of glucose and glucose oxidase, the enzyme-catalyzed reaction consumes O₂ and produces peroxide. Therefore, glucose present in the sample is correlated with a decrease in O₂ concentration and an increase in peroxide concentration, either one of which be detected by a specific electrode. By the method of the present invention, electrical current generated can be measured directly. In the neutral red system, a specific compound in an unknown test sample is tested using cells or enzymes that are capable of oxidizing the compound to generate a detectable current upon oxidation of the compound by the biocatalyst. Therefore, the concentration of the compound can be determined by measuring the electricity generated upon oxidation of the test compound. It is well within the ability of one skilled in the art wishing to detect a particular compound to adapt the method of the present invention to detect the compound by selecting a suitable biocatalyst capable of oxidizing the compound.

Another important application using neutral red provides a method for measuring the chemical oxygen demand in waste water comprising (a) providing an electrochemical fuel cell system comprising an anode compartment and a cathode compartment separated by a cation-selective membrane, wherein each compartment is equipped with an electrode, wherein the electrodes are connected by a wire to a multimeter; (b) placing an anolyte in the anode compartment, the anolyte comprising a suitable concentration of neutral red and waste water comprising or supplemented with a biological catalyst; (c) placing a suitable catholyte in the cathode compartment; (d) allowing the neutral red-mediated conversion of chemical reducing power to electricity; (e) measuring the electrical current generated by the fuel cell system.

The following nonlimiting examples are intended to be purely illustrative.

EXAMPLES

Chemicals and Reproducibility of Results

All chemicals were reagent grade and gases were purchased from AGA Chemicals (Cleveland, Ohio, USA). All individual experiments were repeated two to three times with identical results.

Electrochemical Bioreactor Systems

The ECB system I (40 ml working volume) was used for enzymatic and chemical reduction tests and ECB system II (300 ml working volume) was used for electrical-dependent cultivation of cells. The ECB systems, specially designed for maintaining anaerobic conditions and for growing bacteria, were made from Pyrex glass by the MSU Chemistry Department, East Lansing, Mich., USA. The ECB system was separated into anode and cathode compartments by a cation selective membrane septum (diameter [φ]=22 mm for type I and [φ]=64 mm for type II) (Nafion, Electrosynthesis, Lamcosta, N.Y.); 3.5 Ωcm⁻² in 0.25 N NaOH). Chemicals and metabolites cannot be transferred across the Nafion membrane; only protons or cations transfer. Both the anode and cathode were made from graphite fine woven felt (6 mm thickness, 0.47 m²g⁻¹ available surface area (Electrosynthesis, NY, USA). A platinum wire ([φ] Ω0.5 mm, <1.0 Ωcm⁻²; Sigma, St. Louis, Mo., USA) was attached to the graphite felt using graphite epoxy (<1.0 Ωcm⁻², Electrosynthesis, NY, USA). The electric resistance between anode and cathode was <1 kΩ. The weight of both electrodes was adjusted to 0.4 g (surface area, 0.188 m²) for system I and 3.0 g (surface area, 1.41 m²) for system II. The current and voltage between anode and cathode were measured by precision multimeter (Fluke model 45, Everett, Wash., USA) and adjusted to 0.3-2.0 mA and 1.5 volt for system I, and 1.0-10.0 mA and 2.0 volt for system II, respectively. The electrochemical half oxidation of H₂O was coupled to half reduction of NR (100 μM) and the oxidation of reduced NR was coupled to bacteriological reduction of fumarate. H₂ was not produced under the electrochemical conditions used to reduce NR or MV. For tests in ECB system I, the cathode compartment contained the cell suspension, membrane suspension or solubilized membranes and the anode compartment contained 50 mM phosphate buffer (pH 7.2) and 100 mM NaCl. For growth studies in ECB system II, the cathode compartment contained the growth medium inoculated with A. succinogenes and the anode compartment contained 100 mM phosphate buffer (pH 7.0) and 100 mM NaCl.

Organism and Growth Conditions

A. succinogenes type strain 130Z is maintained at MBI International (Lansing, Mich., USA) (10,39). Bacteria were grown in butyl-rubber-stoppered, 158 ml serum vials containing 50 ml medium with CO₂—N₂ (20% -80%, 20 psi) gas phase, unless stated otherwise. The growth medium A contained the following (per liter of double distilled water): yeast extract, 5.0 g; NaHCO₃, 10.0 g; NaH₂PO₄.H₂O, 8.5 g; and Na₂HPO₄, 12.5 g. The pH of medium was adjusted to be 7.0 after autoclaving. Separately autoclaved solutions of glucose (final concentration 60 mM), and fumarate (final concentration 50 mM) were aseptically added to the medium after autoclaving. Media were inoculated with 5.0% (v/v) samples of cultures grown in the same medium and incubated at 37° C.

Preparation of Cell Suspensions

Bacterial cultivation, harvest and washing were done under strict anaerobic N₂ atmosphere as described previously (39). A 16 h A. succinogenes culture was harvested by centrifugation (5,000×g. 30 min) at 4° C. and washed three times using a 1500 ml solution of 50 mM Na phosphate buffer (pH 7.2) containing 1 mM dithiothreitol (DTT). The washed bacterial cells were re-suspended in 50 mM sodium phosphate buffer with 2 mM DTT. This suspension was used as a catalyst for H₂-dependent and electrical-dependent reduction of fumarate to succinate; and, it was used for cyclic voltammetry and for NR absorption to cells.

Electrochemical Reduction of NAD⁺ or NADP⁺

ECB system I with 1 mM NAD⁺ or NADP⁺ and 100 (ΩM NR or MV was used for electrochemical reduction of NAD⁺ or NADP⁺. The electrode potential and current were adjusted to 2.0 volts and, 1.0-3.0 mA, respectively. Ag/AgCl and platinum electrodes were used to measure the reactants redox potential to check if the reaction was progressing. Generally, the redox potential of a biochemical or electrochemical reaction is measured using an Ag/AgCl electrode (E_(O) ′ of [Ag/Ag⁺], =+0.196 volt) or a Calomel electrode (E_(o) ′ of [Hg/Hg⁺], +0.244) as a reference electrode but it has to be expressed as the potential vs. natural hydrogen electrode (NHE), which is used for thermodynamical calculation of organic or inorganic compounds (e.g., E_(o) ′ of NADH/NAD⁺ is a −0.32 volt and H₂ /2H⁺ is −0.42 volt). A potential measured using Ag/AgCl electrode is converted to potential vs. NHE by adding +0.196 volt to the measured potential (E_(o) ′ vs. NHE=E_(o) ( vs. Ag/AgCl+0.196). Oxygen was purged from the reactants and from the redox dye solution in 50 mM Tris-HCl (pH 7.5) by bubbling with oxygen free nitrogen for 10 min before supplying electricity. The NADH concentration in the reactant was spectrophotometrically measured at 340 mm and calculated using the millimolar extinction coefficient 6.23 mM⁻¹ cm⁻¹ . NADH or NADPH production was confirmed by absorption spectra data at each sampling time.

Preparation of Purified Membranes, Solubilized Membranes and Membrane Free Cell Extract

Cell free extracts were prepared at 4° C. under an anaerobic N₂ atmosphere, as described previously (Van der Werf et al., 1997, Arch. Microbiol. 167:332-342). The harvested and washed cells were resuspended in 50 mM phosphate buffer (pH 7.2) containing 1 mM DTT and 0.05 mg/ml deoxyribonuclease. Cells were disrupted by passing twice through a French Press at 20,000 psi. The cell debris was removed by centrifugation three times at 40,000×g for 30 min. The purified membranes were obtained from the cell free extracts by centrifugation at 100,000×g for 90 min. The supernatant was decanted and saved as the membrane-free cell extract. The brown and clear precipitate was washed twice with 50 mM phosphate buffer (pH 7.2) and re-suspended in the same buffer by homogenization. Solubilized membranes were obtained from membrane fraction by Triton X-100 extraction (Lemire et al., 1983, J. Bacteriol. 155:391-397). Triton X-100 was added to a final 1% (v/v) concentration and, the suspension was incubated for 3 h. Triton-solubilized protein was recovered after removing insoluble debris by centrifugation at 100,000×g and 4° C. for 90 min.

Neutral Red Binding to Cells and Membranes

The absorption of redox dyes to cells and purified membranes was determined by measuring the residual NR and MV in solution after mixing with cells or membrane suspensions for 30 min at 37° C. Bacterial cell suspensions (OD₆₆₀ between 0-3.0) and the purified membrane suspension (0-10 mg/ml protein) were used to analyze redox dye absorption (i.e., binding). NR solutions (50 μM and 25 μM) and MV (100 μM) were used for measuring dye binding to intact cells and membranes. MV (100 μM) was used for cell binding. The cells and membranes were removed from the reaction mixture by centrifugation at 12,000×g for 10 min and by ultracentrifugation at 150,000×g for 20 min, respectively. The NR concentration was calculated using a calibration curve spectrophotometrically pre-determined at 400 nm and pH 7.2, and MV was determined using the millimolar extinction coefficient (578) 9.78 mM⁻¹ cm⁻¹ after reduction by addition of Elepsiden 1.5 mM dithionite at pH 7.2 (Lissolo et al., 1984, J. Biol. Chem. 259:11725-11729). The protein concentration of membrane suspensions was determined by a calibration curve (protein concentration, mg/ml=A₅₉₅×1.3327) using Bradford Reagent (Bio-Rad, Hercules, Calif., USA).

Measurement of Proton Translocation

Proton translocation was measured under an anoxic N₂ atmosphere. H₂-dependent proton translocation by cell suspensions was measured as described by Fitz and Cypionka (Fitz et al., 1989, Arch. Microbiol. 152:369-376). Electrical-dependent proton translocation was measured in an electrochemical bioreactor system designed for measurement of proton translocation. The tube ([φ] 10 mm ID and 90 mm length) with a Vycor tip (ion exchangeable hard membrane, Bas, West Lafayette, Ind., USA) was used as an anode compartment and a graphite rod ([φ] 7 mm×70 mm) was used as an anode, and 0.05 g graphite felt (surface area, 0.0235 m²) was used as a cathode. The pH electrode (Orion 8103 ROSS) was placed in the cathode compartment and was connected to a recorder (Linear) via a pH meter (Corning, 130) that converted the proton pulse into a recordable signal. Cell suspensions were made in KKG solution (pH 7.1) which contains 100 mM KSCN, 150 mM KCl and 1.5 mM glycylglycin and placed in the cathode. The anode contained a 50 mM phosphate buffer with 50 mM KCl as an anolyte. The total volume and working volume of the cathode and anode compartments were 30 ml and 5.5 ml, respectively. The working potential and current between anode and cathode were 2.0 volt and 0.3-0.35 mA for experiments using electrical reducing power and NR. Bacterial cells were cultivated for 16 h in medium A with fumarate-H₂ or glucose. The cells were anaerobically harvested by centrifugation at 5,000×g and 20° C. for 30 min and washed twice with 100 mM KCl. The cells were modified with 100 μM NR to measure electrical-dependent proton translocation and washed again with 100 mM KCl. The washed bacteria (OD,₆₆₀, 10) were re-suspended in N₂-saturated 150 mM KCl. Cell suspensions were allowed to equilibrate for 30 min at room temperature. The incubated cells were centrifuged at 5,000×g and 20° C. for 30 min and re-suspended in KKG solution and then the incubation was continued for 30 min under H₂ atmosphere before the measurement of proton translocation. To measure electrical-dependent proton translocation upon fumarate addition, the cell suspension was incubated in the presence or absence of HOQNO in the cathode compartment under N₂ atmosphere and charged with 2.0 volt electrode potential for 20 min.

Enzyme Assays

Enzyme activity measurements were performed under an anaerobic N₂ atmosphere, as described previously (Van der Werf et al., 1997, Arch. Microbiol. 167:332-342) . The membrane-free extract, purified membrane and solubilized membrane preparations described above were used to assay hydrogenase, diaphorase, and fumarate reductase activities. Fumarate reductase (EC 1.3.) and hydrogenase (EC 2.12.2.2.) activities were measured as described by van der Werf (1997, Arch. Microbiol. 167:332-342), with a Beckman spectrophotometer (Model, DU-650). Diaphorase activity with BV²⁺ and NR⁺ was measured under analogous conditions with hydrogenase using NADH (0.6 mM) instead of H₂ as electron donor (Schneider et al., 1984, Eur. J. Biochem. 142:75-84). The oxidation and reduction of benzyl viologen and NR were spectrophotometrically measured at 578 nm and 540 nm, and the oxidation and reduction of NAD(H) were spectrophotometrically measured at 340 nm. Reduced benzyl viologen was prepared as described previously (Lissolo et al., 1984, J. Biol. Chem. 259:11725-11729). The millimolar extinction coefficient of benzyl viologen (578), NR (540) and NAD(H) (340) were 8.65 mM⁻¹ cm⁻¹, 7.12 mM⁻¹ cm⁻¹, and 6.23 mM⁻¹ cm⁻¹, respectively.

Enzymatic Analysis of Fumarate Reduction Membranes and Solubilized Membrane

Membrane suspensions (3.25 mg/ml protein) and solubilized membranes (3.2 mg/ml protein) were used as the enzyme sources. Serum vials (50 ml) and ECB system I was used for H₂-dependent and electrical dependent reduction of fumarate to succinate, respectively. Anaerobically prepared 50 mM fumarate in 50 mM phosphate buffer (pH 7.2) was used as reactant and catholyte, and 100 mM phosphate buffer with 100 mM NaCl (pH 7.0) was used as anolyte. The reaction was started by the addition of enzyme sources and it was maintained at 37° C. Substrate and product concentrations were analyzed by HPLC (Guerrant et al., 1982, J. Clin. Microbiol. 16:355-360). The influence of HOQNO on fumarate reduction in cell suspensions and membranes were analyzed as follows.

Cell suspensions (OD₆₆₀=4.2) and membrane suspension (2.65 mg/ml protein) were used as the enzyme sources. Serum vials (50 ml) and ECB system I was used for H₂-dependent and electrical dependent reduction of fumarate to succinate, respectively. Anaerobically prepared 50 mM fumarate in 50 mM phosphate buffer (pH 7.2) was used as reactant and catholyte and 100 mM phosphate buffer with 100 mM NaCl (pH 7.0) was used as analyte. 2 μM HOQNO was used as an inhibitor for menaquinone. The reaction was started by the addition of enzyme sources and it was maintained at 37° C. Substrate and product concentration was analyzed by HPLC.

Cyclic Voltammetry

A 3 mm diameter glassy carbon working electrode (BAS, West Lafayette, Ind., USA), platinum wire counter electrode (BAS), and an Ag/AgCl reference electrode (BAS) were used in an electrochemical cell with a working volume of 2 ml. Cyclic voltammetry was performed using a cyclic voltametric potentiostat (BAS, model CV50W) linked to an IBM microcomputer data acquisition system. Prior to use, the working electrode was polished with an alumina/water slurry on cotton wool, and the electrochemical cell was thoroughly washed. Oxygen was purged from the cell suspension, membrane suspension, or solubilized membrane solution by bubbling with oxygen free N₂ for 10 min before electrochemical measurements. Bacterial suspensions (OD,₆₆₀=3.0), membrane suspensions (2.54 mg protein/ml), and solubilized membranes (3.2 mg protein/ml) were used as enzyme sources. The scan rate used was 25 mV/s over the range −0.3 to −0.8 volt 50 mM phosphate buffer containing 5 mM NaCl was used as electrolyte. NR μ100 (M) and 50 mM fumarate was used as the electron mediator and the electron acceptor, respectively.

Growth Analysis

Growth of cells suspended in the medium was determined by measuring the suspensions (optical density at 660 nm), the growth yield of cells absorbed onto the electrode was determined by measuring protein concentration. The protein concentration was converted to optical density using a predetermined calibration curve (bacterial density=protein concentration, mg/ml×1.7556). The cathode, on which the bacteria absorbed, was washed three times, by slow agitation, in 300 ml of phosphate buffer (50 mM, pH 7.0) for 30 min. The bacterial lysate was obtained from electrodes by alkaline treatment at 100° C. for 10 min using 1N-NaOH. After removing cell debris from the lysate by centrifugation at 10,000×g and 4° C. for 30 min, the protein concentration of the bacterial lysate was determined using Bradford Reagent (Bio-Rad, Hercules, Calif., USA), and a predetermined calibration curve (protein concentration, mg/ml=A₅₉₅×1.3327).

Methanogenic Granules Growth and Metabolic Analysis

Methanogenic granules containing mixed cultures of fatty acid-degrading syntrophiles and methanogens were obtained from a bench scale anaerobic sludge reactor fed on a mixture of 50 mM acetate, butyrate, and propionate in MBI International (Lansing, Mich.) (Wu et al., 1993, Arch. Microbiol. 39:795-803 and Wu et al., 1993, Appl. Mirobiol. Biotechnol. 39:804-811). Methanogenic granules were cultivated in PBBM prepared without organic compounds (Kenealy et al., 1981, J. Bacteriol. 146:133-140). The medium was prepared without phosphate, brought to pH 7.2 with NaOH, boiled, sparged with N₂—CO2 (80:20%) or H₂—CO₂ (80:20%), dispensed into 158-ml Wheaton serum vials, sealed with butyl rubber stoppers, and autoclaved. Phosphate, sulfide (0.01%), N₂—CO₂ (80:20%) or H₂—CO₂ (80:20%), and vitamin solution were added after autoclaving. The medium volume was 40 ml, and the initial head space gas pressure in serum vials was adjusted to 30 psi. Media were inoculated with 3.0% (by volume; protein concentration, 1.995 mg/ml) methanogenic granules and incubated at 37° C. All procedures for medium preparation, inoculation, and cultivation were the same as those used for vial cultures except that Na₂S was not added because the medium was electrically reduced. Na₂S (2%) was added to the anode compartment as reducing agent to remove the O₂ generated. NR (100 (M) was added to the cathode compartment as electron mediator. The current and potential between anode and cathode were 0.4 mA and 2.0 volts. CO₂ and CH₄ were analyzed using a gas chromatograph equipped with a carbosphere column and flame ionized detector. The injector and column temperatures were 50° C. and 150° C., respectively, and the carrier (N₂) flow rate was 45 ml/min. Gas samples were removed with a pressure lock syringe. CO₂ consumption and CH₄ production are shown as the percentage of total gas composition in the headspace.

Bacterial Growth and Cell Preparation for Generating Electricity

A. succinogenes 130Z and E. coli K-12 were anaerobically grown for 16 h and 20 h, respectively, in medium A (10 g/l glucose, 5 g/l yeast extract, 8.5 g/L NaH₂PO₄, and 10 g/l NaHCO₃) under an anaerobic N₂—CO₂ (80:20) atmosphere at 37° C. in 150 ml serum vials or under a N₂ (100%) atmosphere in fuel cell system with a pH controller. The inoculum size was 3% (v/v) for both vial and fuel cell experiments. Resting cell suspensions were prepared by harvesting stationary phase cultures at 40° C. by centrifugation at 5,000×g. The cells were washed twice using 50 mM phosphate buffer (pH 7.0) under a 100% N₂ atmosphere. The washed cells were resuspended in 50 mM phosphate buffer (pH 7.0), then dissolved O₂ was removed by gassing with N₂ for 30 min. The cell density was adjusted to OD₆₆₀ 3.0.

Fuel Cell Systems for Growing or Resting Cells

A two-compartment (anode and cathode) electrochemical cell was used as a fuel cell system for microbial electricity production (FIG. 1). When switches one and two are off, there is an open circuit. When switch one is on and switch two is off, a closed circuit is formed. When switch one is off and switch two is on, a closed circuit with external variable resistance is formed. One hundred uM NR or 300 microM thionin were used as the electron mediator. The total and working volumes of each compartment were 1,600 ml and 1,300 ml, respectively. The electrodes, each made of 12 g fine woven graphite felt (0.47 m²/g, Electrosynthesis, NY) were connected to a precision multimeter (Fluke model 45, Everett, Wash.) with a platinum wire ([φ]=0.5 mm, <1.0 Ωcm⁻²; Sigma, St. Louis, Mo., USA) using graphite epoxy (<1.0 Ωcm⁻², Electrosynthesis, NY). Anode and cathode compartments were separated by a cation-selective membrane septum ([φ] 70 mm, Nafion, Electrosynthesis, NY). The self-electric resistance of the fuel cell system between the anode and cathode was approximately 1,000Ω. The resistance was adjusted using variable resistance for controlling current production, but it was not adjusted for measuring maximum potential or current production. The current and voltage between the anode and cathode were measured by a precision multimeter (Fluke model 45, Everett, Wash.). The electrochemical half-reduction of ferric ion (as potassium ferricyanide, E_(o)(=+0.36 volt)—which was re-oxidized by O₂ (E_(o)′=+0.82 volt) was coupled to neutral red or thionin half-oxidation which was, in turn, reductively coupled to bacterial oxidative metabolism. In the fuel cell system using resting cells, the bacterial cell suspension (OD₆₆₀, 3.0) in 50 mM phosphate buffer (pH 7.2) containing 100 microM NR or 300 microM thionin, and 100 mM phosphate buffer (pH 7.0) containing 50 mM ferricyanide were used as the anolyte and catholyte, respectively. In the fuel cell system using growing cells, medium A containing a fresh bacterial inoculum was the anolyte; the catholyte was the same as for resting cells. During experiments, complete anoxygenic conditions were maintained in the anode compartment by gassing with 100% N₂ for 30 min before operation at N₂ flow rates of 0.8 ml/min. The trace oxygen contained in the N₂ gas was removed in a furnace filled with pure copper fillings at 370° C. The cathode compartment was oxygenated by constant air bubbling and stirring. The anode compartment was maintained at pH 7.0 using an automatic pH controller (New Brunswick Scientific Co., model pH-40, Edison, N.J.).

Current Production by Dye Chemical Oxidation Coupled to NADH Oxidation

A small chemical fuel cell system (total volume 50 ml; working volume 30 ml) consisting of an anode and cathode compartments equipped with 0.3 g fine woven graphite felt electrodes and a cation-selective membrane septum (Ω20 mm, Nafion, Electrosynthesis) was used. A 100 microM NR solution in 50 mM phosphate buffer (pH 7.0) and 100 mM phosphate buffer (pH 7.0) containing 50 mM ferricyanide were used as the anolyte and catholyte, respectively. Oxygen was completely removed from the anode compartment by N₂ gassing for 30 min before adding NADH. The concentrated NADH solution in 50 mM phosphate buffer (pH 7.0) was previously gassed with N₂ to remove O₂.

Cyclic Voltametry

A 3 mm-diameter glassy carbon working electrode, a platinum wire counter electrode, and an Ag/AgCl reference electrode (all from BAS, West Lafayette, Ind.) were used in an electrochemical cell with a 3 ml working volume. Cyclic voltametry was performed using a cyclic voltametric potentiostat (model CV50W, BAS) linked to an IBM personal computer data acquisition system. Prior to use, the working electrode was polished with an aluminum/water slurry on cotton wool, and the electrochemical cell was thoroughly washed. Oxygen was purged from the reactant by bubbling with oxygen-free N₂ for 10 min before electrochemical measurement. The scanning rate used was 25 mV/s over the range −0.3 to −0.8 volt. A 50 mM phosphate buffer containing 5 mM NaCl was used as the electrolyte. One hundred μM NR and 100 μM NAD were used as the electron mediator and acceptor, respectively.

Generation of Electricity Using Anaerobic Sludge

The anaerobic sludge was obtained from the East Lansing sewage treatment plant (MI, USA). The fresh anaerobic sludge was settled under a N₂ atmosphere for one day to remove solid particles. The supernatant (1,200 ml) was used as biocatalyst and anolyte for the fuel cell system, to which 3 g/L glucose was added as energy source. The catholyte was 100 mM phosphate buffer (pH 7.0) containing 50 mM ferricyanide.

Results

Electricity Generation by Fuel Cells

The E_(o)′ values of the electron mediators used for converting the reducing power generated by microbial metabolic oxidation to electricity are important determinants of the maximum electricity amount that can be generated in microbial fuel cells. Chemical properties of artificial electron mediators (i.e., NR and thionin) with those of natural electron mediators (i.e., NAD⁺ and menaquinone) are shown in Table 1. The electron driving force generated from using NR is far greater than from thionin when the redox dye is coupled to an oxidant (i.e., ferricyanide) in a chemical or microbiological fuel cell. This difference is due to differences between the E_(o)′ values for NR and thionin. Consequently, the delta E_(o) generated from NR or thionin oxidation coupled to ferricyanide reduction is 0.645 volt (NR) and 0.296 volt (thionin). These delta E_(o) values are the theoretical maximum potentials produced in fuel cells using these electron mediators.

Results of experiments performed demonstrate the superiority of NR over thionin as an electron mediator and that reduced NR is able to donate electrons to the electrode for electricity production in a microbial fuel cell. FIG. 2 shows that the use of NR as an electron mediator in a chemical fuel cell generates higher current than that obtained using thionin, and that the current produced depends on the NADH

TABLE 1 Redox mediators, their structural formula, redox potentials (E_(o)′), and maximum absorbance wavelength (λ_(max)). Structural formula Redox mediator E_(o)′ (V) λ_(max)

Neutral Red −0.325 540

Thionine +0.064 598

Menaquinone −0.074 260/280

NAD⁺ −0.32 340

concentration used. Arrows indicate the addition of 1 (circles) or 3.5 (squares) mM NADH. At low NADH concentrations the current was quite low. Although thionin reduction was faster than NR reduction when using NADH as the reductant, the mediator oxidation rate at the electrode is rate-limiting, because more current was produced with NR as the electron mediator.

Cyclic voltammograms of a NR solution in the presence or absence of NAD⁺ show that NR oxidation (upper) and reduction (lower) peaks did not shift during twenty scanning cycles in the absence of NAD⁺ (FIG. 3A) . Both peaks increased upon NAD⁺ addition (FIG. 3B). NAD⁺ enables more electrons to pass unidirectionally from the electrode to NR to NAD and from NADH to the electrode via NR.

FIG. 4 compares the currents and potentials generated from glucose by E. coli resting cells in a glucose (10 g/L) fuel cell with either 100 uM NR (circles) or 300 uM thionin (squares) in closed circuit (current) (A) and open circuit (potential) (B) configurations. Arrows mark (1) the addition of the electron mediator; and (2) conversion to open circuit. Under the anaerobic conditions used, higher current and potential levels were produced with NR than with thionin as the electronophore. In control experiments under aerobic conditions, significant levels of current or potential were not detectable because NR and thionin cannot oxidize NADH through the electron transport system since O₂ is a much better electron acceptor (i.e., it has a much more positive E_(o)′ value than the two electron mediators). Under anaerobic conditions, E. coli normally couples NADH oxidation with reduction of either fumarate to succinate, acetyl CoA to ethanol, or pyruvate to lactate. These reactions are inhibited in the presence of NR in the fuel cell, and electricity is produced in lieu of these normal reduced metabolic end products.

Previous investigations (Allen et al., 1993, Appl. Biochem. Biotechnol. 39-40:27-40 and Thurston et al., 1985, J. Gen. Microbiol. 131:1393-1401) have shown in microbial fuel cells using thionin as the electron mediator, that both current and potential drop when the resting cells are depleted of glucose. We performed experiments to determine what maximal electrical productivities and stabilities can be generated by resting E. coli cells from different glucose concentrations in a fuel cell with NR as the electronophore. Table 2 shows the effect of glucose concentration on the maximal electrical productivities and stabilities in an open circuit versus a closed circuit, with and without a 120 ohm external resistance. The maximal current, potential, and electrical energy produced by the fuel cell were proportional to the glucose (i.e., fuel) concentration. The maximum current and coulombic yields obtained from glucose using NR as the electronophore far exceeded those obtained with thionin in other investigations (Dealney et al., 1984, Chem. Tech. Biotechnol. 34B:13-27).

Previous studies (Roller et al., 1984, J. Chem. Tech. Biotechnol. 34B:3-12 and Bennetto et al., 1985, Biotechnol. Lett. 7:699-105) on microbial fuel cells with thionin as the electron mediator were only performed with resting cell suspensions (i.e., cells harvested after growth had ended). Using NR (100 uM) as the electronophore, we compared the

TABLE 2 Effect of initial glucose concentration on electrical productivity and stability of a microbial fuel cell using E. coli resting cells and NR as the electronophore. Closed Circuit with a 120Ω Resistance Open Circuit Closed Circuit Electrical Glucose Potential Current Potential Current Potential Current Coulomb Electrical Stability (mM) (volt) (mA) (volt) (mA) (volt) (mA) (amp/sec) Energy (J) (hr) 11.1 0.58 0.0 0.02 1.2 0.46 0.5 57.6 26.5 32 55.5 0.65 0.0 0.04 5.6 0.57 3.6 1049.76 598.4 81 111 0.85 0.0 0.05 17.7 0.62 4.8 2039.04 1264.2 118

electrical productivities (i.e., current and potential) of A. succinogenes growing cells (FIG. 5A) and resting cells (FIG. 5B) in a glucose (10 g/L) microbial fuel cell under anaerobic conditions. with (FIG. 5). Control experiments (FIG. 5A) showed that the growth yield and rate (squares) were much higher in the absence of NR when no electricity was generated (triangles) than in the presence of NR. The electric current (open circles) and potential (closed circles) generated increased with cell growth. The potentials generated by growing and resting cells were similar, whereas the current produced by resting cells was significantly higher (about 2-fold) than that produced by growing cells. The specific current produced per mg cell protein per hour was calculated at 10 h for growing cells (1.235 mA/mg protein/hr) and at 2 h for resting cells (2.595 mA/mg protein/hr) when the glucose levels were high. A total of 68 coulombs was produced by growing cells at 20 h (after glucose was depleted); whereas the resting cells had produced 90 coulombs at 4 h.

Similar experiments were performed using anaerobically grown E. coli cells in the presence or absence of electrical generation. Electrical generation dramatically decreases growth yield, ATP yield and metabolite production (Table 3). Table 4 compares substrate consumption, growth and electricity production by exponential versus stationary phase E. coli cells. These data indicate that significantly more electricity is produced by stationary phase cells than by exponential phase cells. This result was expected because significant reducing power is required for cell growth that cannot be directed to electricity generation.

TABLE 3 Comparison of anaerobic metabolism of E. coli during anaerobic growth in the presence or absence of electrical generation^(a). Glucose Cell Ysub Theoretical Electricity Growth Consumption Mass (g cell/mol Products ATP yield Energy Condition (mM) (g/L) substrate) (mM) (mol/mol sub) (J/mol sub) Without 60.6 3.07 50.12 11.93 7.07 — Electricity Generation With 66.3 1.4088 22.082 8.88 2.57 1320.0 Electricity Generation ^(a)Data was determined after 20 h of growth in medium A with 100 μM neutral red in a standard fuel cell.

TABLE 4 Comparison of substrate consumption and electricity production by anaerobic E. coli exponential phase versus stationary phase cells in a fuel cell using neutral red as electronophore^(a). Exponential Phase Cells Stationary Phase Cells Electricity Electricity Glucose Energy Glucose Energy (J/mol Consumption Cell Mass (J/mol sub) Consumption Cell Mass sub) 45.1 mM 1.74 g/L 100.8 15.5 mM 0.214 g/L 1207.7 (7.52 mM/hr) (0.29 g/L/hr) (2.59 mM/hr) (0.035 g/L/hr) ^(a)Data for exponential phase cells is from 0-6 h after inoculation. Data for stationary phase cells is 12-18 h after inoculation. Conditions: medium A with 100 μM neutral red in the standard fuel cell system.

Experiments were initiated using anaerobic sludge to test its potential as a catalyst for electricity generation in a fuel cell with NR as the electronophore. FIG. 6 shows the effect of glucose addition on the current and potential generated by the sewage sludge, as well as the maximum current produced in a closed circuit configuration versus the maximum potential produced in an open circuit configuration. The numbered arrows connote the conversion from open to closed circuit with a 2.2 kohms resistance (1); addition of 3 g/L glucose (2); conversion from closed to open circuit (3); and the conversion from open to closed circuit without external resistance (4). The electrical productivity of the glucose fuel cell using sewage sludge as the catalyst was calculated to be a total of 370.8 C (G of 162.82 J).

We have shown here that NR serves as a superior electronophore or electron mediator than thionin in microbial fuel cells using glucose as fuel. Furthermore, we have shown that resting cells generate more electricity than growing cells, and that mixed cultures such as sewage sludge can be robust catalysts for electricity generation in fuel cells utilizing NR as the electron mediator.

FIG. 7 summarizes our working model explaining E. coli (or A. succinogenes) metabolic properties in fuel cells using NR as the electron mediator versus during normal (A) versus electrogenic glucose metabolism (B) of E. coli or A. succinogenes in a fuel cell with NR as the electronophore. Cell growth, ATP synthesis, and reduced end product formation decrease in relation to the amount of electricity generated. In the presence of NR, cell growth is significantly reduced and NADH is oxidized via NR-mediated electrical generation in lieu of producing normal reduced end products (i.e., succinate, lactate, and ethanol). Cells still generate ATP by substrate-level phosphorylation (i.e., acetate kinase) but grow slower because they cannot generate ATP by electron transport-mediated phosphorylation (i.e., fumarate reductase).

NR is superior to thionin as an electron mediator because it enhances both the rate of electron transfer (current) and the yield of electrons transferred (coulombic yield). The highest current (>17 mA) produced in a microbial fuel cell using NR was significantly higher than that achieved previously with thionin as the electron mediator (Roller et al., 1984, J. Chem. Tech. Biotechnol. 34B:3-12 and Allen et al., 1993, Appl. Biochem. Biotechnol. 39-40:27-40); it is, however, still low in electrical terms. There may be potential applications for low-power DC microbial fuel cells such as to maintain telecommunications in remote areas including outer space.

The present invention is not limited to the exemplified embodiments, but is intended to encompass all such modifications and variations as come within the scope of the following claims. 

We claim:
 1. A method of promoting reductive processes in a biological system comprising the steps of: (a) providing an electrochemical bioreactor system having a cathode compartment equipped with a cathode and an anode compartment equipped with an anode, the cathode and anode compartments are separated by a cation selective membrane, wherein the cathode and anode are connected by a conductive material to a power supply; and (b) placing a suitable amount of neutral red and a biological catalyst in the cathode compartment, wherein at least a portion of the neutral red is reduced, and wherein the biological catalyst comprises cells selected from the group consisting of Actinobacillus succinogenes and methanogenic archea.
 2. A method of promoting reductive processes in a biological system comprising the steps of: (a) providing an electrochemical bioreactor system having a cathode compartment equipped with a cathode and an anode compartment equipped with an anode, the cathode and anode compartments are separated by a cation selective membrane, wherein the cathode and anode are connected by a conductive material to a power supply; and (b) placing a suitable amount of neutral red and a biological catalyst in the cathode compartment, wherein at least a portion of the neutral red is reduced, wherein the biological catalyst comprises cells and wherein the cells comprise methanogens.
 3. The method of claim 2, wherein reduced neutral red promotes growth of the methanogens or reduction of carbon dioxide to methane.
 4. The method of claim 2, additionally comprising a step wherein cell growth and methane production takes place in the absence of molecular hydrogen (H₂).
 5. A method of promoting reductive processes in a biological system comprising the steps of: (a) providing an electrochemical bioreactor system having a cathode compartment equipped with a cathode and an anode compartment equipped with an anode, the cathode and anode compartments are separated by a cation selective membrane, wherein the cathode and anode are connected by a conductive material to a power supply; and (b) placing a suitable amount of neutral red and a biological catalyst in the cathode compartment, wherein at least a portion of the neutral red is reduced, wherein the biological catalyst comprises cells and wherein the cells comprise succinogenic bacteria, and wherein the bacteria reduces fumarate to produce succinate.
 6. A method of promoting reductive processes in a biological system comprising the steps of: (a) providing an electrochemical bioreactor system having a cathode compartment equipped with a cathode and an anode compartment equipped with an anode, the cathode and anode compartments are separated by a cation selective membrane, wherein the cathode and anode are connected by a conductive material to a power supply; and (b) placing a suitable amount of neutral red and a biological catalyst in the cathode compartment, wherein at least a portion of the neutral red is reduced, wherein the biological catalyst comprises an isolated enzyme that uses cells NADH or NADPH as a cofactor, wherein the cathode compartment comprises NAD (H) or NADP (H) wand an oxidized substrate for the enzyme and wherein the reduced neutral red transfers electrons to NAD⁺ or NADP⁺, and wherein the enzyme comprises an alcohol dehydrogenase, the oxidized substrate comprises an aldehyde or ketone, and a reduced product is produced, wherein the product comprises an alcohol. 